Techniques for producing solid materials from the vapor phase are generally referred to as deposition technologies, which are distinguished from melting/solidification technologies in which solid materials are produced from the liquid phase. The properties of solid materials produced from the vapor phase can be varied over a much wider range than the same materials produced from the liquid phase. Deposition technologies can be generally classified as: (1) physical vapor deposition, including evaporation, sputtering, and ion plating; (2) chemical vapor deposition; (3) ion implantation; (4) electrodeposition; and (5) plasma spraying, including detonation gun technology.
While the novel deposition technology of the present invention has some similarities with plasma spraying and detonation gun technologies, it comprises the first embodiment of a new category of deposition technologies. The process of the present invention can be classified as a vapor phase or fine particulate coating technology based on the use of an inductively coupled plasma torch. There is no known prior embodiment of a similar vapor phase or fine particulate deposition process.
Inductively coupled plasma torches have been employed primarily in optical emission spectroscopy. For that purpose, the construction and operation of plasma torches are well developed. See, for example, Fassel and Kniseley (1974), Anal. Chem., 46(13):1110A-1118A; Fassel (1978), Science, 202:183-191; Douglas and Houk (1985), Prog. Analyt. Atom. Spectrosc., 8:1-18; and U.S. Pat. Nos. 3,324,334 and 4,501,965.
Orifice arrangement for molecular beam sampling in spectroscopy applications was analyzed in Pertel (1975), Intern. J. Mass Spect. and Ion Phys., 16:39-52. These arrangements involved aligned orifices in the tips of cones. The effect of supersonic velocities on beam scattering is discussed in relation to four conditions illustrated in FIG. 6 (page 49), wherein the molecular beam accelerates to a supersonic velocity after passing through the first orifice. For optimized beam sampling with minimum scattering, Pertel recommends having the Mach disk in effect attached to the orifice tip of the second cone. A somewhat related arrangement is described in Douglas U.S. Pat. No. 4,501,965 for a spectrographic mass analyzer.
The use of a plasma torch for growing refractory crystals by a melting/solidification technique has been described by T. B. Reed (1961), J. Appl. Phys., 32:2534-2535. Particles of the refractory material are carried by a monatomic gas through the center of an inductively coupled toroidal plasma. The material is melted by plasma heating, and deposited from the liquid phase on the growing crystal. A similar method and apparatus is described in Reed's U.S. Pat. No. 3,324,334.
More recently an inductively coupled plasma system operating at reduced pressure has been proposed for the production of silicon carbide powder: Hollabaugh et al. (1983), J. Materials Sci., 18:3190-3194. The silicon carbide is formed by gas phase chemical reaction by injection of silane, methane, and hydrogen into the plasma flame, the siiicon carbide being recovered as a powder.
Arc-type plasma torches have been proposed for use in coating processes. See U.S. Pat. Nos.: Matvay, 3,324,114; Muehlberger, 3,839,618; and Guyonnet, 4,146,654. Arc torches may be provided with multiple orifices (Giannini et al. U.S. Pat. No. 2,922,869 and Guyonnet U.S. Pat. No. 4,146,654). Arc-type torches utilize electrodes between which the arc is struck for generating the plasma. These electrodes may be a source of contamination in the plasma flame. Some arc torches use consumable wire as electrodes. These wires can melt at different rates and/or form large droplets of molten material. Further, the powder is introduced into the plasma and the material interacts directly with the plasma flame and surrounding environment.
As illustrated by the cited Muehlberger patent, if the target to be coated is located within a vacuum chamber, the arc-type torch is inserted into the vacuum chamber. This is a virtual necessity because in arc-type plasma torches the material to be deposited is principally on the surface of the arc. Therefore, unless the arc torch is inserted into the vacuum chamber, only a small quantity of the material can be drawn into the deposition chamber. However, such insertion of the torch into the vacuum chamber will result in a lowering of the plasma temperature. This precludes the use of very high plasma temperatures where the plasma must be operated at or near atmospheric pressure.